1. Field of the Invention
The present invention relates to a scanning optical apparatus and, more particularly, to a scanning optical apparatus, for example suitably applicable to apparatus such as laser beam printers (LBPs) or digital copiers, that is arranged to effectively intercept part of a laser beam from a light source means by a shade member having a shade portion formed in a predetermined shape, thereby decreasing the spot diameter, increasing the focal depth, and suppressing the negative effect of side lobe on an image.
2. Related Background Art
In the conventional scanning optical apparatus used in the laser beam printers, the digital copiers, or the like the laser beam (light beam) emitted from the light source means is optically modulated according to an image signal. Then the optically modulated laser beam is deflected on a periodic basis by a light deflector, for example comprised of a polygon mirror, and an imaging optical system having f-.theta. characteristics focuses the beam in a spot shape on a photosensitive recording medium. The beam thus scans the recording medium to effect image recording thereon.
FIG. 1 is a schematic view of major part in a main scanning cross section of a scanning optical apparatus of this type.
In the same drawing, the laser beam (light beam) L emitted from the light source means 31 is converted into a nearly parallel beam or into a converging beam by a collimator lens 32, a stop member 33 shapes the size of beam cross section of the beam, and the shaped beam is incident into a cylindrical lens 34. Among the parallel beam incident into the cylindrical lens 34, rays in the main scanning cross section are emergent in the state of parallel rays as they are. In the subsidiary scanning cross section perpendicular to the plane of FIG. 1 rays are converged to be focused as an almost linear image on a deflecting surface (or reflecting surface) 35a of the light deflector 35. After that, the laser beam deflected by the deflecting surface 35a passes through an imaging optical system (f-.theta. lens) 36 to be converged on a photosensitive drum surface 37, and the light deflector 35 is rotated in the direction of arrow A to effect optical scanning in the direction of arrow B (i.e., in the main scanning direction) on the photosensitive drum surface 37. This results in recording an image on the photosensitive drum surface 37 being a recording medium.
In these years, the laser beam printers, digital copiers, or the like are being required to achieve higher resolution and higher quality of image and, especially for meeting requirements for high resolving power and for tone reproduction affluent in half tones (intermediate tone levels), it is necessitated to further decrease the spot diameter on the photosensitive drum surface being a surface to be scanned.
Here, the above spot diameter d is expressed by the following equation. EQU d=.alpha.F.lambda. (a)
In Eq. (a),
F: F-number of image-forming beam (optical system), PA1 .lambda.: wavelength of laser beam, PA1 .alpha.: constant. PA1 F: F-number of image-forming beam (optical system), PA1 .lambda.: wavelength of laser beam, PA1 .beta.: constant. PA1 light source means; PA1 deflecting means for deflecting a beam emitted from said light source means; PA1 optical means for guiding the beam emitted from said light source means to said deflecting means; PA1 a stop member, provided in an optical path between said optical means and said deflecting means, for shaping a beam size of the beam having passed said optical means, said stop member having an aperture portion; PA1 imaging means for guiding the beam deflected by said deflecting means onto a surface to be scanned; and PA1 a shade member, provided in the optical path between said stop member and said deflecting means, for intercepting a beam traveling in a central part of said stop member. PA1 a shape of a shade portion of said shade member satisfies the following conditions: EQU Ps(eff)&lt;Pm(eff)+0.2022/{Pm(eff)+0.1718}-0.1716, EQU Pm(eff)&lt;Ps(eff)+0.2022/{Ps(eff)+0.1718}-0.1716, EQU 0&lt;Ps(eff)&lt;1, and EQU 0&lt;Pm(eff)&lt;1, PA1 where Pm(eff) is an effective shade length ratio in a direction of deflection of said deflecting means with respect to the aperture portion of said stop member and Ps(eff) is an effective shade length ratio in a direction perpendicular to the direction of deflection of said deflecting means with respect to the aperture portion of said stop member.
Therefore, in order to further decrease the spot diameter d, it is necessary to decrease the F-number of image-forming beam or to decrease the wavelength .lambda. of laser beam, as seen from Eq. (a). However, a decrease in the F-number of image-forming beam will result in an increase in the beam width of laser beam L outgoing from the collimator lens 32. It thus becomes necessary to increase the effective diameter of collimator lens 32. Such a collimator lens has the problem of increase in the manufacturing cost, because the number of lenses used therein increases and because higher adjustment accuracy thereof is required.
In addition, since the width of the beam incident to the deflecting surface (reflecting surface) of light deflector 35 also becomes greater, the effective area of the deflecting surface must be increased, too. The manufacturing cost of such a light deflector increases and the weight of the light deflector itself also increases. Thus, a very large load is exerted on a motor for rotating the light deflector. This will result in such various problems as an increase in the cost of the motor and an increase in consumption power.
As for shortening of the wavelength .lambda. of laser beam, the cost is normally still high of lasers creating visible light, which are used for some high-grade machines, but are not suitable for cheap laser beam printers.
On the other hand, the focal depth D of the laser beam focused in the spot shape on the photosensitive drum surface is expressed by the following equation. EQU D=.beta.F.sup.2 .lambda. (b)
In Eq. (b),
As seen from Eq. (b), the focal depth D tends to become smaller (shallower) as the F-number of image-forming beam is made smaller or as the wavelength .lambda. of laser beam is made shorter in order to decrease the spot diameter. This necessitates decreasing the curvature of field of the f-.theta. lens (imaging optical system). As a result, design becomes complex and higher accuracy is required for processing of the f-.theta. lens itself. Further, higher assembly accuracy is necessitated for each optical component, which will result in a problem of a large increase in the cost.
In order to solve this problem, there are methods proposed, including a method for generating a Bessel beam of the first kind of order 0 to decrease the spot diameter and to increase the focal depth, for example as described in Japanese Laid-open Patent Application No. 5-307151 or in Japanese Laid-open Patent Application No. 6-148545, a method for intercepting the side-lobe part of an eccentric Bessel beam by a slit member, and so on.
Creation of the Bessel beam, however, required use of, for example, a conical prism and a phase/amplitude filter or the like, which tended to complicate the whole apparatus. The method for intercepting the side-lobe part of eccentric Bessel beam by the slit member tended to complicate adjustment.